So, after building most of my GS to Proficient by playing all my players in the 48-72 minute range, this week all but one fell to Strong even though they all played in the 48-72 minute range (mostly 56-64 minutes).
The exceptions: I had one guy who was Proficient who only played 48 minutes and stayed at Proficient.
I had another guy who was Respectable, but played 75 minutes and increased to Strong.
There must be quite a bit of randomness with Game Shape management.
It appears that the best you can do is hit your minutes goal of 60 and see what happens.
Not the hugest issue in the world, but definitely something to be aware of.